Our View of a Decelerating Magsail

byPaul GilsteronMarch 25, 2015

Yesterday’s post looked at the question of starship detection. But the paper by Ulvi Yurtsever and Steven Wilkinson that I discussed actually focused on a highly specific subset of such observations, the case of an artificial object moving at such high gamma factors that the ship’s velocity was over 99 percent of the speed of light. It may be that such things become possible to sufficiently advanced civilizations, but if they do and we observe them, we will be doing something akin to what Richard Carrigan does when he looks for Dyson spheres. Hunting a relativistic starship between galaxies is a kind of interstellar archaeology.

What I mean is that if any of the researchers now looking for observational data of advanced civilizations turn something up in, say, M31, that construct will be so far away from us in both space and time that we might well be studying the ruins of an ancient culture. I made this case not long ago in an essay called Distant Ruins for Aeon magazine. This is a different kind of SETI, one in which communication is almost certainly not an issue, just as would be the case if we detected the signature of a starship in the intergalactic deep.

Science fiction that looks at starship detection usually takes a more active stance. In Arthur C. Clarke’s wonderful Rendezvous with Rama (1973), it is an early warning system to detect dangerous asteroids that initially notices the starship humans will name Rama. Further observations help nail down a trajectory that comes from interstellar space and will return there, while the detection of a rapid rotation and, later, photographs of the object as a perfect cylinder make it clear that this will be our species’ first encounter with an alien vessel.

And then there’s The Mote in God’s Eye by Larry Niven and Jerry Pournelle, published the following year, in which astronomers detect a starship in the form of a laser-beamed lightsail. It’s an older technology in the world of the novel, for in this future scenario humans have already produced a star drive that eclipses the much slower sail method. Observed changes in the brightness and color of a Sun-like star in a binary system (the star is called the Mote for reasons explained in the novel) turn out to have been caused by the operation of the laser system beaming the craft toward the Earth.

Here the scene is the bridge of a human starship as the crew discusses the observations that lead to the discovery that they are dealing with an alien technology, specifically its laser beaming system:

“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye [the other star in the binary pair], and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”

“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”

“Light,” Potter said firmly.

“Light sail!” Rod shouted in sudden realization…”

Niven and Pournelle had vetted their lightsail concept with Robert Forward at a time when the idea was just gaining traction thanks to the latter’s work in the journals.

In both novels, the starship detection has huge consequences for our species as the craft in question is entering human space. When I went back and looked at Robert Zubrin’s 1995 paper on starship detection, I remembered that he came up with interesting figures for different kinds of starships. An antimatter photon rocket would produce gamma ray emissions that would be undetectable at visual wavelengths, but Zubrin found that based on his assumptions on an arbitrarily chosen 1,000,000 ton craft, an exhaust with an effective irradiated power of 1,800,000 TW would be produced. He went on to describe its detectability:

Such an object at a distance of 1 light-year would be seen from Earth as a 17th magnitude light source, and could be detected on film by a first class amateur telescope. The 200 inch telescope on Mount Palomar could image it at 20 light years, and the Hubble Space Telescope at a distance of about 300 light years… Since at least for the upper-end telescopes considered, the number of stellar systems within range is significant (100,000 stars are within 200 light years of Earth) this approach offers some hope for a successful search. The light from the photon rocket could be distinguished from that of a dim star by the lack of hydrogen lines in the rocket’s emissions.

Here again we’re dealing with a vast volume of space but a distance of no more than 200 light years in any direction. But beyond the visual spectrum, Zubrin discusses a variety of scenarios, noting that radio waves may be emitted from a starship due to plasma interactions with the deceleration field of a magnetic sail, or the confinement field of a plasma drive engine. Now the detection distances grow greater. The plasma drive engine‘s electron and ion cyclotron radiation could be detected on the ground by radio telescopes. Magsails produce electron cyclotron radiation with frequencies of tens of kHz and ion cyclotron radiation with frequencies of tens of Hz. No ground detection here, but the magsail radiation would be apparent to receivers of sufficient size working outside the Earth’s atmosphere.

Image: One configuration of a magsail as envisioned by Steve Bowers on the Orion’s Arm site. This design uses multiple superconducting loops for maximum braking effect against the interstellar medium. Credit: Steve Bowers.

The low frequency magsail radiation is made to order for large antennae in space, making it the easiest starship configuration to detect:

It can be seen that the magsail radiation of a characteristic fusion starship being decelerated from a cruise velocity of 0.1c could be detected by a 6 km orbiting antenna from a distance of 400 light years, while that emitted by a characteristic antimatter photon rocket in its deceleration phase could be seen as far away as 2,000 light years. There are about 100,000,000 stellar systems to be found within the latter distance. This extended range detection capability combined with magsail radiation’s unique time-dependent frequency spectrum appears to make a search for magsail radiation the most promising option for extraterrestrial starship detection.

As mentioned yesterday, Al Jackson has been considering the question of starship detectability for some time, and in mid-2014 published a preliminary paper on the matter describing his findings. It’s Jackson’s work that, even more than Yurtsever and Wilkinson, pushes the boundaries of speculation the furthest, or as Al puts it in the paper, “…the methods used to attain relativistic speed, using high-energy astrophysical processes, are far out in the tail of the distribution of speculation…” And he adds this tantalizing thought: “Is there a ‘Wow!’ signal lurking in the non-standard parts of the SETI electromagnetic spectrum? Starships braking in a dense interstellar region are attractive possible observations.”

Tomorrow I want to look at some of these speculative starship ideas and the kind of signatures they might throw, as per Jackson’s paper. For today, the Zubrin paper is “Detection of Extraterrestrial Civilizations via the Spectral Signature of Advanced Interstellar Spacecraft,” Progress in the Search for Extraterrestrial Life, ASP Conference Series Vol. 74 (1995). Available online.

This series is very timely; I have, just today, been mulling over using magsails to decelerate vehicles from a relatively low speed (100s of km/s) into the Oort Cloud. It’s good to know I could detect them coming…

This reminds me of my now late mother on UFOs. She would roll her eyes when she heard people say aliens were looking at our military bases . In the 80s at the Chicago air show she pointed to the Stealth Bomber and said There is your UFO . That had me thinking years that only magic could hide an Alien arrival !.
I also thought she would have enjoyed the Orion flight but she would have been annoyed we not going to do anything for years. But she enjoyed even space station live in her 90s. Her father was an engineer in Germany so that is where she got her enjoyment of space and her eye rolling skepticism that intersteller spaceships were popping up all over

Whoaaa! A “million tonne spaceship”?!
Let’s look at the energies involved: to get 1 kg to a speed of 0.1c = 3 x 10^7 m/s requires E = 0.5mv^2 = 4.5 x 10^14 J (ideally, with no inefficiency losses). To decelerate to 0 requires the same amount, so the ‘trip cost’ is close enough to 10^15 J for every kilogram . So the cost for a 1,000,000 ton = 10^9 kg craft is 10^26 J. Where is this energy coming from (For comparison, the entire annual energy consumption of the Earth in 2008 was 5 x 10^20 J (1.4 x 10^5 TW-h; http://en.wikipedia.org/wiki/World_energy_consumption).)

Yet another reason this site is pure platinum. Up until yesterday I was only aware of a couple of ways we might detect nearby spacecraft (other than one appearring right on top of us!) so thankyou Paul et al for reminding me that there are plenty of thougts and ideas in this area coming from several directions… seems like a lot of those ideas have passed me by recently. Thankyou for fixing that.

As an aside… the magsail configuration pictured above looks to me to be an ‘Apollonian Gasket’-type fractal… very nice to look at :)

Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion’s Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.

The Ancient Greeks thought that Sirius’s emanations could affect dogs adversely, making them behave abnormally during the “dog days,” the hottest days of the summer. The Romans knew these days as dies caniculares, and the star Sirius was called Canicula, “little dog.”

And quite oddly there is some debate about the change of its color during recorded history (red controversy). Of course all this can be attributed to mistakes and exaggerations, but still… its odd, isn’t it?

On a second note, there seems some kind of development in fiber lasers going on at the moment, championed by Lockheed Marten’s 30kw laser slicing a motor block. I am not sure if the recent development in fiber lasers could have implications for beamed propulsion, it may be worth a look.

There’s plenty of energy available for million tonne starships in fusion fuels. If we wanted to use antimatter, then supply becomes more of an issue using current means of making the stuff, though there’s a glimmer of hope that we might find a much more efficient means via Condensed Compact Objects (CCOs), which astronomer Marshall Eubanks has highlighted. CCOs are ‘quark matter’ or similar ultra-dense chunks that formed during the Big Bang and/or as ejected debris from supernova. Via a process involving the Colour force, a beam of regular matter fired into a CCO can very efficiently produce antimatter (~50% efficiency), so if a civilization reaches the K-II level and has 4E+26 W to utilize, then making a megatonne of antimatter is a relatively trivial affair.

The main problems with anti matter is firstly the reaction to form gamma rays is that they are hard to focus if not usefully impossible due to their neutral properties and secondly is the containment. Using them to ignite other fuels is better but still leaves us with the tricky problem of containment.

@Michael: Gamma rays sure are hard to contain. On the other hand, AFAIK, the initial product of antimatter annihilation is mesons, which could conceivably be deflected before they decay to gamma rays. Black holes are also interesting for matter to energy conversion. Natural black holes often have tightly focused jets coming out of their poles, which ought to serve as a proof of principle for a potential engineering solution.

I believe Eniac meant this, the accretion disc is at a very high temperature ‘plasma’ and the magnetic fields created are responsible for the ‘polar’ outflows but we still don’t know exactly why and how. Whatever the process they are truly powerful events. Aliens could ride these outflows, but I doubt any planet within easy reach would be habitable after taking a hit from one of these beams, I mean matter travelling at near the speed of light.

@Ron: I don’t think we really know how omnidirectional Hawking radiation is after it extricates itself from the incredibly intense fields near the event horizon of a small, magnetic black hole. And if it does take an accretion disk, maybe one could be engineered. It would be good not to have the hole evaporate, anyway. I understand they go with quite a bang.

@Michael: Ideally we’d have a small, portable black hole that we’d feed with matter. We’d polarize the hole so one of the jets is suppressed. A relatively weak external magnetic field could weaken the internal field at one pole and strengthen it at the other, causing the desired jet imbalance. May not be feasible, of course. We are talking science fantasy, here….

”There’s plenty of energy available for million tonne starships in fusion fuels. ”

‘Adam: This is mere handwaving.’

There is about 24000 tons of deuterium per cubic kilometre of sea water on earth, therefore there is plenty of fusion fuel available. We may not like the neutron radiation given off by the D-D reaction though.

‘It would be good not to have the hole evaporate, anyway. I understand they go with quite a bang.’

If we send in some charged particles like electrons to charge the hole it could explode as they over power gravity, which could come in handy as all the energy would be released at once as very fast charged particles that entered the hole which can directed. As I understand it the electron is indivisible and excess electrons should survive in the hole, all theory I may add :)

‘Ideally we’d have a small, portable black hole that we’d feed with matter. We’d polarize the hole so one of the jets is suppressed. A relatively weak external magnetic field could weaken the internal field at one pole and strengthen it at the other, causing the desired jet imbalance. May not be feasible, of course. We are talking science fantasy, here….’

If we fed the little black holes with mater plasma jets would be emitted at its ‘poles’ just applying a magnetic field to push/pull the magnetic field created could be used to control its movements.

Eniac: “I don’t think we really know how omnidirectional Hawking radiation is after it extricates itself from the incredibly intense fields near the event horizon of a small, magnetic black hole.”

What intense fields are those? Also, a BH has no magnetic field. A BH is a black body radiator.

“And if it does take an accretion disk, maybe one could be engineered.”

Yes, but the devil is in the details. You’ll want to do some careful calculations before thinking this is a viable approach to propulsion. See further below.

“It would be good not to have the hole evaporate, anyway. I understand they go with quite a bang.”

The ‘traditional’ way of BH propulsion is to harness Hawking radiation of somewhat lever its gravitation. But as far as evaporation goes it depends on how fast and how soon. A hot small BH can last a long time. I think the idea is all nonsense but we should at least keep the nonsense accurate.

“We’d polarize the hole so one of the jets is suppressed.”

You can’t polarize a hole. You can perhaps play games with infalling matter.

Michael: “If we send in some charged particles like electrons to charge the hole it could explode as they over power gravity, which could come in handy as all the energy would be released at once as very fast charged particles that entered the hole which can directed.”

No. The electrons increase the hole’s mass and increase the electric field. The BH will not suffer indigestion. It would also take a substantial quantity of electrons for a measurable effect. By ‘substantial’ I mean an inconceivably enormous quantity of electrons.

Just thinking about how to control small black holes, simple particles fired at very high velocities into them will be enough to move them about via the conservation of momentum. The benefit would be little radiation as the particle are swallowed up quickly.

The “magnetic hole” I am hypothesizing about is one that produces a magnetic dipole field of (macroscopically) ordinary strength. Whether this field is an intrinsic property of the hole (which I think you claim is impossible), or generated by whatever is right outside the event horizon does not matter. We know there are large holes with magnetic fields (and jets). I am simply assuming (for lack of better knowledge) that small holes may have that too.

Now, if you calculate the field of a dipole of ordinary strength at a distance comparable to the radius of the hole (which is really, REALLY tiny), you get the intense fields I am talking about. I believe the dipole field goes with 1/r^2. Assuming it is ordinary (say, 1 mT) at 1 m, now tell me what it is at a femtometer or so. Really, really strong, I’d say. By “polarizing the black hole” I mean applying an external field that will pull at the dipole field. Not sure what the effect of that near the event horizon would be, but in the absence of detailed knowledge about how jets are produced, we may as well assume that they are sensitive to such polarization. I would be so bold as to say that this last part is the most fantastic assumption I am making. The others seem comparatively reasonable.

Actually, the most fantastic bit is to try and imagine how a small hole can be fed at all, given that its radius is way smaller than most particles, if I recall correctly. By small I mean one having ordinary mass (say. tons), rather than astronomical mass as normal holes do.

Eniac: “We know there are large holes with magnetic fields (and jets).”

Sorry, but no. Any magnetic field is a property of the accretion disk, and especially how its properties are transformed by BH interaction. You don’t have to believe me; there is lots of readily available material, even on Wikipedia.

“Actually, the most fantastic bit is to try and imagine how a small hole can be fed at all, given that its radius is way smaller than most particles, if I recall correctly.”

That’s why I said ‘the devil is in the details’. You could swat a micro BH with your hand and have nothing at all happen, not to you or the BH. But wear some protective gear (including on the hand you’re swatting with) since the micro BH will be very hot and spitting out all sorts of interesting particles.

‘What intense fields are those? Also, a BH has no magnetic field. A BH is a black body radiator.’

The magnetic field is wrapped around the black hole, the part that enters the BH can’t get out.

‘No. The electrons increase the hole’s mass and increase the electric field. The BH will not suffer indigestion. It would also take a substantial quantity of electrons for a measurable effect. By ‘substantial’ I mean an inconceivably enormous quantity of electrons.’

The strength of the electromagnetic force is 10^40 greater than that of gravity, the addition of the masses of electrons is smaller than the effect of the charge by a considerable margin. So in order to balance the force of gravity you would need ~1/10^44 (1/1800 mass of proton +) mass of electrons, that is a very small number. The reason the universe is so electrical neutral is due to the huge force that it has, but we can play around with it locally.

‘Actually, the most fantastic bit is to try and imagine how a small hole can be fed at all, given that its radius is way smaller than most particles, if I recall correctly. By small I mean one having ordinary mass (say. tons), rather than astronomical mass as normal holes do.’

It would be problematic but electrons and some particles are almost point like with a limit set on their diameters due to their masses (i.e the Schwarchild radius, electron ~10^-57 m). We are still hundreds of times away from looking at the ‘structure’ of electrons energy wise.

Paul, are we experiencing a script time warp? I could have sworn that Ron S paragraph (April 1, 2015 at 21:34) was not there when mine was posted onto the page or I am I going mad or slightly more mad…

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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